Re: [malware-list] [RFC 0/5] [TALPA] Intro to a linux interface for on access scanning

tvrtko.ursulin
2008-08-06T16:04:22Z

Greg KH <greg@kroah.com> wrote on 06/08/2008 16:25:55:

> On Wed, Aug 06, 2008 at 10:37:06AM +0100, tvrtko.ursulin@sophos.com
wrote:
> > Greg KH wrote on 05/08/2008 21:15:35:
> >
> > > > > > Perf win, why bothering looking for malware in /proc when it
can't
> > > > > > exist? It doesn't take longer it just takes time having to do
> > > > > >
> > > > > > userspace -> kernel -> userspace -> kernel -> userspace
> > > > > >
> > > > > > just to cat /proc/mounts, all of this could probably be
alliviated
> > if we
> > > > > > cached access on non block backed files but then we have to
come
> > up with
> > > > > > a way to exclude only nfs/cifs. I'd rather list the FSs that
> > don't need
> > > > > > scanning every time than those that do....
> > > > >
> > > > > How long does this whole process take? Seriously is it worth
the
> > added
> > > > > kernel code for something that is not measurable?
> > > >
> > > > Is it worth having 2 context switches for every open when none are
> > > > needed? I plan to get numbers on that.
> > >
> > > Compared to the real time it takes in the "virus engine"? I bet
it's
> > > totally lost in the noise. Those things are huge beasts with
thousands
> > > to hundreds of thousands of context switches.
> >
> > No, because we are talking about a case here where we don't want to do
any
> > scanning. We want to detect if it is procfs (for example) as quickly
as
> > possible and don't do anything. Same goes for any other filesystem
where
> > it is not possible to store arbitrary user data.
>
> See previous messages about namespaces and paths for trying to figure
this
> kind of information out in a sane way within the kernel.

How are namespaces and pathnames relevant to exclusions based on
filesystem *type*? It is not about checking for /proc but checking if the
filesystem name is proc, sysfs, ..

> > > > > > In kernel caching is clearly a huge perf win.
> > > > >
> > > > > Why? If the cache is also in userspace, it should be the same,
> > right?
> > > >
> > > > In kernel cache has 0 context switches for every open. Userspace
> > > > caching has 2. Every open has to block, switch to the context of
the
> > > > userspace client/cache, get that decisions, and then switch back
to
> > the
> > > > original process.
> > >
> > > Again, compared to what? If you in userspace are doing big complex
> > > things, such an overhead is trivial.
> >
> > Again similar thing as above - In case of a cache we are not doing
complex
> > things.
>
> Except for the overhead of keeping a cache :)

As small as possible - just one extra field in the inode struct.

And really you are talking about a different thing again and moving the
argument around. I answered you why your argument about putting a cache in
userspace since we do so much there already is wrong, because we don't do
a lot (anything really) in userspace for vast majority of opens if we have
this totally simple in-kernel caching scheme.

> > So I think you can't argue that because scanning is slow everything
> > else has to go to userspace. On a typical running system scanning is
> > exceptional and everything else benefits from being in the fast path.
>
> I really can not judge as we have not seen an implementation yet.

Did you mean "_I_ have not seen an implementation yet"? Because Eric
posted it so you can have a look at your leisure.

Tvrtko

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